This invention relates to cross-linked hydrophilic polymers which are suitable for use as a carrier for medicaments and other active ingredients; as hydrophilic membranes for separation processes; bandages for wound treatment; body implants, e.g. artificial veins and coatings on glass, metal, wood or ceramics, and in particular, for use in applications where strength of the polymer article and high permeability to water are required simultaneously.
It is known to produce sparingly cross-linked, water-insoluble but hydrophilic polymers which can be used as carriers for biologically active, at least slightly water-soluble substances by copolymerization of a major amount of hydrophilic mono-olefinic monomers and a minor amount ranging between 0.01 and 15% of said mono-olefinic monomers, of a low molecular weight cross-linker. As mono-olefinic monomers, particularly monoesters of acrylic or methacrylic acid with polyfunctional alcohols, such as ethyleneglycol mono-methacrylate, and as cross-linking agents particularly diesters of said acids with said alcohols, such as ethyleneglycol bis-methacrylate are used and the copolymerization is carried out in the presence of water, see U.S. Pat. No. 3,220,960 or a water-free system, see U.S. Pat. No. 3,520,949. Low molecular as well as macromolecular, water-soluble substances, such as polyethyleneoxide mono-methacrylate together with a minor amount of the corresponding bis-methacrylate have been used, U.S. Pat. No. 3,220,960, as monomers and cross-linking agents. The water-insoluble, but hydrophilic copolymers and the process for their production have been modified in several directions and adapted to specific purposes, e.g. the production of soft contact lenses, U.S. Pat. No. 3,220,960 and Reissue No. 27,401, and the copolymerization in the presence of linear polyamide resin in order to improve or modify the mechanical properties of shaped bodies formed from the obtained polymers, U.S. Pat. No. 3,520,949. However, in all modifications low molecular-weight poly-olefinic cross-linking agents, especially ethyleneglycol bis-methacrylate, were used in very small to moderate amounts never exceeding 20% of the amount of the mono-olefinic monomer. Though the copolymers of the type described above could be modified to comply with the requirements of several different modes of using them, the mechanical properties in either the unswollen, i.e. water-free, or the swollen, i.e. equilibrium state with water, could not be satisfactorily adapted to all modes of uses.
It is known that hydrophilic polymers whose major constituents are monoesters of acrylic acid and methacrylic acid and a bi-functional alcohol have glass transition temperatures or softening points between 55.degree. C. and 80.degree. C. For this reason said prior art articles are brittle and glassy in the dry state at temperatures below 55.degree. C. After equilibration in water the articles of said prior art become soft and somewhat pliable but also weak with respect to their flexural properties. In addition, said prior art articles are very susceptible to tearing shear forces if they are knicked or injured in any way.
In order to avoid the undesirable weak characteristics of articles produced by said prior art, a medium made of a stronger polymeric material is used as physical support, or the pre-polymerized mixture is filled with an insoluble material such as silica gel. These techniques, although they afford a certain amount of cohesive strength (the hydrogel material acting as a glue), produce articles which are still susceptible to glassy fracture in the dry state and shear fracture in the swollen state within the interstitial regions of the article. By the same token, additional of fillers to the prepolymer modify the diffusion properties and water permeability of the articles.